Many beneficial substances, including many therapeutic agents, require long-term delivery to a target site of action to be optimally effective. Well-known examples include drugs that need to be administered for extended periods of time to a patient. Many extended release formulations have been developed for this purpose. A common issue with all of these formulations is that the drugs administered need to be stabilized in the formulation for the duration of the shelf-life of their dosage form, in addition to the stabilization required during the extended release period.
In many cases, drugs are more stable in a dry or solid formulation than in a dissolved state, hence formulations having a solid formulation during shelf life are often preferred. In some instances, the solid drug may be dispersed in a liquid, resulting in a liquid formulation comprising a solid drug.
However, in order to be released from their dosage form, drugs almost always rely on some type of diffusional mechanism, which inherently requires the drugs to be in solution. Therefore, many dosage forms rely on the uptake of water after administration to a patient to bring the drugs from a solid form into solution, prior to release from the dosage form.
One type of dosage form that has been developed to address the issue of extended release of therapeutic agents is that of implantable drug delivery devices, in which a reservoir holding a drug formulation is combined with a release rate controlling mechanism, such as a release rate controlling membrane. In many instances, when a solid or dry formulation, like a powder, is filled into such a reservoir, a quantity of air is included in the reservoir. As was mentioned above, many of these dosage forms rely on the uptake of water to bring their drugs from the solid form into solution, essentially requiring that air inside the reservoir be replaced with water. Oftentimes, this will require simultaneous mass transport of water into a device and air out of the device. For those dosage forms that do not allow for such simultaneous transport, proper hydration of the formulation inside the reservoir may be impeded. One type of dosage form where this can be the case is implantable drug delivery systems having a capsule encapsulating a reservoir containing a therapeutic agent in a dry form, and a release rate controlling membrane based on nanopores. In many cases, the reservoir and the nanopores will contain an amount of air in addition to the therapeutic agent, and mass transport of interstitial fluid into the reservoir after implantation may be impeded by the presence of the air. Therefore, additional technologies are desired that allow for proper hydration in such dosage forms.